Trying to make sense of Surrey migration

I’ve had a few discussions recently with people about the routes and landmarks birds use to migrate over Surrey. Over the years I’ve heard various theories about different sites having their own reasons for being on a flight path. It’s a topic that interests me deeply. This post tries to make vague sense of it.

Four things are important to remember when reading this post:

1. In Surrey, the consensus is birds migrating any long distance in autumn do so either south, south-east or west (occasionally east). In spring, it’s more varied. 
2. Birds move over the countryside on a broad front during peak migration (i.e. on a big day you can witness it anywhere, whether your back garden or atop of a viewpoint).
3. There’s no detailed analysis of weather conditions in this post, which are of course very important. It would make for too long a post.
4. All this thought is purely speculative!

The presumed key migration landmarks in Surrey will be described, followed by a look at routes/junctions. Finally, a list of suggested vis-mig hot-spots is proposed. A rough conclusion is that the North/South Downs are probably a quite strikingly undervalued migration route; the Greensand Ridge too, but less so. Ideally, in time, this page will be edited and annotated as further opinions and discussions take place.

A topographic map of south-east England. The North Downs are obvious, the Greensand Ridge less so. Even this zoomed out, the Dorking Gap and Wey-Arun Gap are clear.

The red line is the North Downs and the blue line the Greensand Ridge.

Landmarks


North Downs

This 156-mile-long chalk ridge extends to Surrey from Dover, Kent (itself first landfall for migrants). A look at a topographic map shows that any migrant bird arriving in Kent in autumn and looking to head west/inland can’t avoid the North Downs. Many birds arriving to East Sussex in the autumn from the south or east, that are heading north or west, will eventually hit the Greensand Ridge then the North Downs. Any birds arriving in Surrey from the north and heading south, perhaps having travelled across Britain or come in off the east coast somewhere, will hit the North Downs, too.

Already, a very straightforward take is that a vantage almost anywhere on the North Downs should produce decent vis-mig. It’s likely that birds arriving from across the Channel to Kent will track the North Downs north and west, as they filter inland. These westerly movements tend to involve thrushes and finches. On a ‘classic’ October or November vis-mig day, the North Downs should deliver.

Southerly or south-easterly movements often involve earlier migrants that have come from the north – hirundines, pipits and wagtails being some examples. For these birds, the North Downs perhaps act more as a barrier then a trail to follow. Some, especially hirundines, probably do use it as a trail, but perhaps to filter south-east to Kent. However, it is likely hirundines, pipits and so on prefer to use the gaps in the North Downs to continue south to the coast. There are two in Surrey, both cut out by rivers – the River Mole at Dorking (the Dorking Gap) and the River Wey at Shalford. These fittingly lead us onto landmark section number two.

River Mole and River Wey

The River Thames is an undeniable fly-way/entry point for many birds (indeed, plenty that move over Surrey probably entered here). However, in a Surrey vis-mig sense, it’s beyond the remit of this post. The River Mole and River Wey are unique for both cutting through the North Downs. They are clear and obvious corridors. Both run in a north-south direction for some part, but the Wey is notable for running vertically for a particularly long distance.

There’s absolutely no doubt birds track the Wey as a result (even if, south of where it cuts through the North Downs, it's unclear how much further they follow it). Indeed, back in the Unstead SF glory days, it was theorised that all the good birds seen (often flying over) were because of the north-south running Wey as both a straightforward route but also as a cut in the North Downs, which loom over the site to the north. Indeed, back in said halcyon days, a couple of years of dedicated vis-mig watches yielded counts of some species that remain as county records/highs.

I’m sure the Mole offers a similar route. I’m less familiar with the area, but long-serving local birders are certain things pass south through the Dorking Gap, before tracking the North Downs or Greensand Ridge west or south. The Osprey we had from Leith Hill tower on Saturday did exactly that. And so, with the mention of the tower and Greensand Ridge, on to landmark number three.

Greensand Ridge

The Greensand Ridge runs along the north of the High Weald. In Surrey, it contains the Surrey Hills Area of Outstanding Natural Beauty. While not as striking as the stretch of North Downs in the county, it still offers an obvious trail of hills with a few, often very small gaps, from Haslemere in the far south-west to Leith Hill (it continues east into Kent but is notably lower east of Leith Hill). Except for the river valleys, the crest of the escarpment is almost continuous along its length, all the way to Kent.

There is one large gap, however, between Hascombe Hill and Winterfold where the old Wey-Arun canal ran. When looking on a topographic map, this couple of mile wide gap – the ‘Wey-Arun Gap’ – is the most obvious and largest one in the Greensand Ridge. Importantly, it also joins with the River Wey gap in the North Downs.

In essence, the Greensand Ridge likely offers a similar trail as the North Downs. Perhaps, even, and certainly in inclement conditions, birds funnel through the very obvious gap between the Greensand Ridge and North Downs. It’s probable wind direction and weather dictate whether birds use the North Downs, Greensand Ridge or both.

This other topographic map of south-east England shows the possibility for a corridor/funnel effect between the North Downs and Greensand Ridge, as well at the edges of each to serve as trails on their own.

Routes and connections


Interestingly, all three of the above landmarks are linked. And such links surely offer the best migration routes. There are a few combinations (with North Downs as A, the Mole Gap as B1, the Wey Gap at Shalford as B2 and Greensand Ridge as C).

A+B1+C: incredibly, at Dorking, the North Downs, Greensand Ridge and River Mole all meet – a veritable junction of inland migration! This space and the various points in the wider area (covered later) likely offer the best position for vis-mig in Surrey.

The Dorking area (A+B1+C) is an impressive junction of landmarks however you look at it. The blue line is the River Mole, the red line the North Downs and the yellow line the Greensand Ridge. Within this map, there are plenty of places with potential for productive vis-mig.

A+B2: the River Wey cuts through the North Downs at south Guildford/Shalford quite strikingly, as a presumably attractive north-south corridor.

A+C: there is a ‘double gap’ effect south of the area mentioned above (A+B2) to the Wey-Arun Plain, the large gap in the Greensand Ridge. On a map, it almost serves as a giant south/south-east corridor, cutting through both the North Downs and Greensand Ridge.

While not as impressive a junction, A+B2 is a clear north-south funnel. A+C, the Wey-Arun Gap, is a large space cutting through both the North Downs (red line) and Greensand Ridge (yellow line). Thus, A+B2 and A+C probably tie together in some way, with birds probably following south-east from A+B2 to the Wey-Arun Gap.

Prime positions


As mentioned earlier, anywhere in Surrey can produce excellent vis-mig – totals from relatively flat/poorly positioned areas down the years have proved that. However, if one takes all the above into account, then there are a few areas that would seem to offer better hope than others. I will break them down in A+B1+C, A+B2 and A+C order.

A+B1+C

Th Dorking mega-junction surely has lots to offer …

Leith Hill

This has to be the best place in Surrey for watching migration. Only relatively recently, Leith Hill tower has been the focus of dedicated vis-mig watches – it’s in an absolutely prime position: from the top, you should see birds come through the Dorking Gap, tracking the North Downs and/or the Greensand Ridge.

The panorama is incredible and it is for this reason that I think such a long list of inland rarities have been seen from or over Leith Hill: Arctic and Great Skuas, Gannet, Honey Buzzard, Common Scoter, Little Gull and Brent Goose to name a few accepted records (Rough-legged Buzzard and Black Stork other credible sightings).

I do think the tower suffers from its height, namely in that it's quite sensitive to weather, in particular fog. As a result, I think some birds will often migrate lower, especially during certain weather and those moving north to south, such as pipits (which I think will then track the gap between the North Downs and Greensand Ridge or merely fly below tower height).

Birds arriving from the west, though, are probably less inclined to do this. So, when the weather is conducive for birds moving at tower height, it is currently the premier site in Surrey for vis-mig.

In the top photo (courtesy M Phelps) taken from Leith Hill tower, the River Mole, North Downs and Greensand Ridge can all be seen (as outlined in the map below) above the fog.

Ranmore Common/Denbie’s Hillside

I have only been to Denbie’s a couple of times. The first was to twitch a Wryneck, the most recent during the Mole Valley Bird Race this summer. However, I think this has the greatest potential of any un-watched migration spot in Surrey and would be hands-down the best site with coverage.

It sits on the North Downs, overlooking the Dorking Gap and the Greensand Ridge – each landmark in one package. I wish I lived closer! Should there be a vantage at the corner of the path at roughly TQ 1593 5078, above Denbies, then it should deliver. Plus, there’s the bonus of many more Wryneck-type things on the deck that too have reached this junction.

Box Hill

On the other side of the Mole is Box Hill. It probably offers a similar set up to Ranmore – perhaps even better – as the Mole is closer and you aren’t looking at all east. However, having never been, I can’t comment too much on its potential. Surely, though, it is similar to that of Ranmore/Denbies.

Betchworth

Another interesting spot is Betchworth. It would offer a similar overhead junction to that of Leith Hill, but is lower (possibly a good thing) and also on a wind gap in the Greensand Ridge.


A+B2

On paper, the River Wey gap in the North Downs looks like an important corridor.

The Chantries

I’d say this has the best potential out of the A+B2 sites. West facing, it sits on the east side of the Wey Gap at Shalford/south Guildford, with the river just below. Most of the incredible Unstead flyovers would have probably been seen from here. Is there a vantage though?

St Catherine’s Hill

On the other side of the Wey (so looking east) is St Catherine’s Hill. This has been studied a little from a migration perspective and probably has good potential. It is, perhaps, a little low at 67 metres, and with the continuation westward of the North Downs right next to it, could be overshadowed …

Artington/Onslow Downs

This has to have potential as it’s on the North Downs itself. Henley Fort looks like a good shout. I’ve done a couple of watches from this stretch of downs further to the west and thrushes certainly track west along the south facing side.


A+C

Jack of all trades, master of none?

Thorncombe Street

The corridor that cuts through the North Downs and then the Greensand Ridge from north to south-east – the Wey-Arun Plain – is used by migratory birds. I know this because I have spent several years watching it. However, because of the curious shape of the land in the area and the addition of the Thorncombe Street Valley, there are several spots from which to watch; perhaps not all that useful.

Generally, the area on the west of the gap, around Hascombe Hill (New Barn and Tilsey Farm), is best, and sees birds clearly moving south and west depending on the time of year. Birds either track the south of the Greensand Ridge, having crossed the Wey-Arun Gap, or track the side of the east end of the Thorncombe Street Valley, having come down the large corridor at Shalford/south Guildford. Hurst Hill, further north, would doubtless be good too but is off limits.

Add in a couple more vantages overlooking the Thorncombe Street Valley, and the fact none of any of the vantages are perfect, then you have a jack of all trades, master of none effect. It’s good and varied, but not the outright best at all. That said, the corridor effect definitely seems to be favoured by north-south things like Meadow Pipits – on big days for them here there has sometimes been no action at Leith Hill.

This birds-eye photo, taken from a plane looking south from Staines Reservoirs (courtesy D Stubbs), shows the 'wall' of the North Downs with, from this angle, a fairly clear gap: the Wey-Arun Gap.

Hascombe Hill

This needs to be watched as it sits on the corner of the Wey-Arun Gap and also the south side of the Greensand Ridge. The area around the Stone Circle offers an excellent view and (whisper it), vis-mig would probably be better here than on my patch.

Winterfold

This, on the east side of the Wey-Arun Gap, would probably be better than both the above but
vantages are few and far between, limited to small gaps in the heavily wooded hillside.



Conclusions


All of the above could be a load of nonsense! But also, given it’s based on the thoughts of several people, there could be some value. Each site mentioned would likely be productive in some way. Many areas not mentioned doubtless would too. The only way to find out is to pick a spot and watch. Over the last few years, there have been obvious correlations with Thorncombe Street and Leith Hill watches, yet also disparities, both in species and numbers.

Ultimately, there is far more that we don’t know about migration – especially in Surrey – than we do. You may have read this and thought ‘well that is all wrong, because in my experience X,Y and Z’ or ‘but what about site/area X, Y or Z’. If so, please let me know. This page will hopefully become a work in progress – annotations, corrections and additions are sought after.